Free piston machine



llg- 8, 1939 R. PATERAs PESCARA 2,168,829

FREE PIsTo'N MACHINE Filed July 14, 1937 2 sheets-sheet 1 lI-III MHQQW'Aug. 8, 1939 R. wmasms PESCARA FREE PISTON MACHINE Filed July 14, 1937 2Sheets-Sheet 2 g I y J5 J1 gl 20 y A 9 E3? 4? 4 .5' 6 Ja Z4 w 2.5

47 5' t '94 M fnl/enfan- VFaI//foae/ws santas etant aidt@ FREEPilS'iiiiN MACYFEPNE Etant Pateras Pescara, Paris, France applicationeen ie, isst,- seifiai No. iea'eae En France duly 17,1936

17 @Batman (Ci. 23M@ The present invention relates to free -piston enceto the accompanying drawings, given meremachnes at least a portion ofthe power of ly by way of example, and in which: which is employed tocompress a gas. It is more Fig. 'l is a diagrammatical view showing., inespecially, although not exclusively, concerned partial longitudinalsection, a motor-compres- 6 with free piston self generating machines,that sor of the free piston type made according to 5 is to say machinesof the type above referred the present invention;

to in which at least a portion of the compressed Figs. 2 and 3 arediagrams showing curves gas is utilized for the feed and the scavengingrelating to the operation of a motor-compressor of the engine cylinder.made according to the present invention and oi 10 The object of thepresent invention is to proan ordinary motor compressor, respectively;10

vide a machine of this kind which is better Fig. 4 is a diagrammaticalview showing, in .adapted to meet the requirements of practice partiallongitudinal section, an auto-generator than machines of the same kindused prior to with free pistons made according to the invenn the presentinvention. tion; The essential ieature of the invention con- Fig. 5 is adiagrammatical view showing, in 15 sists, while devising the machines inquestion partial longitudinal section, an auto-generator in such mannerthat compression' takes place in of the free piston type embodyinganother tea-i: at least two stages, in dimensioning said mature of theinvention; chines in such manner that the compression Fig. is a viewanalogous to Fig. 5, showing ratio of the iirst stage is suilicientlylow in order a modification. 20 that the volumetric ratio of Said stage,and According to the present invention, the motor therefore thevolumetric ratio of the whole of compressor is devised in such mannerthat comthe machin@ may have a Value which iS high andi pression takesplace in several stages, for inlittle variable when the dischargepressure of stance, as it will be supposed inthe following the machineVaries. description, in two stages,V which will be re- 25 Anotherfeature of the inventio relating speetively called pre-compressionstage", and more especially to auto-generating machines of "compressionstage", the kind above referred to, consists in devising According tcthc essential feature cf the inthe elements 0f the machine in Suchmanner vention, instead of endeavouring to obtain, in that the path ofthe gases .during their comeach of said stages. rates of compressionwhich 30 pression and until they enter the engin@ Cylilare substantiallyequal, as it is generally done, der is in a single directlOn. as ShortaS DOSSible I determine the various characteristics of the andpreferably substantially parallel to the axis machine in Such manner(especially the clear- 0f the machina ance spaces and the bores of thecylinders) that A third feature 0f the present' inventiim. Which theratio of compression of the pre-compression 35 relates'mol'e @SpeciallyaliihOllgh nOS exclusively stage is suiilciently low in order that thevolto machines of the type above referred to workumcmc efficiency cfsaid stage and consequently ing With Variable pressures and in Whichmost 'that of the whole of the machine may reach a 1 of the compressionWork takes place during the value which is high and uttie variable whenthe 40 return Strkes 0f the reciprocating elements discharge pressure ofthe compression stage is 40 consists, while making use, for ensuring therevaried, turn strokes of said reciprocating' elements, 0f Thevolumetric ratio of/a compressor is cnarvariable feed pressure Pneumaticacilmulatilsf acterized, in a general manner, by the ratio in providingmeans for proportioning, at least approximately, the quantity of fuelintroduced l 45 into the engine cylinders, or at least the limits V0within which this quantity can be varied for each working pressure, tothe feed pressure of of the suction stroke of its reciprocating elesaidaccumulators or to any pressure proporments and of the total stroke.. Itional to the latter. l From a practicalv point of view, 1t 1s therefore50 f Other features of the present invention will advantageous toincrease the volumetric efiiresult from the following `detaileddescription ciency of a compressor if'it is desired to reduce of somespecific embodiments thereof. its weight and its volume for a givenoutput, Preferred embodiments of the present inventhat is to say toincrease its power per unit of tion will be hereinafter described, withreferweight. 55

In the case of compressors including a crankshaft, it has been possibleto increase their volumetric etliciency by considerably reducing theclearance space, a thing which is not possible with motor-compressors ofthe free piston type, because the clearance spaces of these lastmentioned compressors are relatively important and their variations ofoutput are determined by variations of the discharge stroke.

It is known that the length of the suction stroke V1 is the smaller asthe volume of the clearance space and the discharge pressure aregreater. It will be readily understood, then, that, if steps are takenas just above explained for effecting, in a motor-compressor of the freepiston type, a pre-compression of sufficiently low ratio, it ispossible, while keeping clearance spaces of volumes ensuring a goodworking of these machines, to obtain suction strokes V1 whichapproximate the total strokes Vo,:since the mass of air imprisoned atlow pressure in said clearance spaces is quickly expanded.

Although it is possible to consider making the pre-compression and thecompression above referred to in distinct vessels, it seems that it ismore advantageous, for reasons of construction, to make both operationsin the same cylinder, on either side 'of a compressor piston, working onthe double action principle in said cylinder.

For this purpose, it is advantageous to .have recourse to thearrangement disclosed by Fig. 1 of the drawings.

In this case, the motor-compressor is essentially provided with twomotor pistons I, synchronized in any suitable manner, for inst-ance by alever 38 connected to the ends of two connecting rods 39 and 40, thesepistons moving in a common cylinder 2. 'I'hese pistons I are each rigidwith a compressor piston 3 moving in a distinct cylinder 4.Precompression and compression take place respectively in the chambersof variable volume 5 and 6 located on either side of said compressorpiston 3, which then works on the double action principle in saidcylinderA l, said chambers being for instance connected together by aconduit 1.

I provide the pre-compression chamber 5 with at least one intake valve8. The compression chamber 6 is fitted with at least one discharge valve9. Between the two chambersyfor instance at the outlet of conduit 1 intochamber 6, I provide a valve Ill.

Finally, in' order to form compensating cushions II, I provide systemsof a cylinder I2 vand a piston I3 one of the elements of which, forinstance piston I3 is rigid with the movable parts, to wit pistons I and3,(and the other element consisting of an extensionof the end ofcylinder 4.

'I'he diagram of Fig. 2 permits if understanding the operation of themotor-compressor thus established and the diagram of Fig. 3 permits rofcomparing its operation to that of an ordinary compressor, of the samestroke compressing air to the same final pressure pi.

On the diagram of Fig. 2, a, b, c, d represents the pre-compressioncycle and c, e, f, IJ the compression cycle. It will be understood that,if the pre-compression ratio is relatively low, the curve c, d whichrepresents the expansion of the air imprisoned is clearance space e1existing, at theV end of the pre-compression stroke, at the end ofchamber' is of small length and of a shape which varies but little whenthe nal pressure pi varies.

It follows that the length of the suction stroke K1 Vo will also varybut little when said final pressure pi varies, the variations of thelatter producing chiefly, in said curve, a modification of the expansioncurve f b corresponding to the expansion of the air imprisoned in theclearance space 10 e2 remaining at the end of the compression operationin chamber 6.

The diagram of Fig. 3 shows that, for the same final pressure pi, thesuction stroke V1 of the ordinary compressor is muchv smaller than thesuc- 15 tion stroke V1 of the compressor of the same geometrical strokemade according to the invention. It is further visible that thevolumetric emciency V,v V0 of the ordinary compressor varies Within widelimits when the curve f' b representing the expansion of the airimprisoned at the end of the compression in the clearance space ez isdisposed 25 toward the right as a consequence of an increase of thedischarge pressure pi.

Thus, by way of example, if it is desired to make a motor compressor forwhich the pressure pi would be equal to 5 kg., the clearance space 30 e2is such that 82- Vif-0.20

and the volumetric eillciency is equal to 0.96, 35'

0x V0 40 is vequal'to 0.20..

If now it is desired to make, no longer a motor compressor of the freepiston type, but an auto-generator of the same type, in which at least aportion of the compressed air is used for scavenging the enginecylinder, I proceed in the following manner:

Preferably, this machine is devised ln accordance with the main featurelof the invention so that the compression of air takes place in at least5o two stages the iirst of which has a ratio of compression which isrelatively low, whereby, for the reasons above stated with reference tothe motorcompressor, the volumetric emciency of said machine ishigh andlittle variable when the final compression pressure varies, this lastmentioned pressure varying as a function of the load of the receivingapparatus, especially turbines, cooperating with the auto-generator.

Furthermore, advantageously, according to another feature of theinvention, which can be used independently of that above set forth, Idevise the elements of the machine vin such manner that the path of airlthrough the stages of compression and until it enters the enginecylinder is in a single direction, as short as possible and preferablysubstantially parallel to the axis of said cylinder, so as to eliminatelosses of power resulting from reversing of the direction of flow of airor to pressure drops in bent conduits and accordingly to improve theeiiiciency of the machine.

For this purpose, and for instance as shown by the drawing (Fig. 4) Iprovide, `in the walls ofv the engine cylinder 2,' on the one handintake 75 `ports I6 capable, when uncovered by one of the engine pistonsclose to its outer end of its reciprocating motion, to connect theengine cylinder with the inside of a uidtight casing I5 surrounding saidcylinder 2. The compressed air is introduced into this casing as itwillbe more fully explained in the followingdescriptlon.

On the other hand, I providey in the walls of cylinder 2, exhaustports'. capable, when they are opened by the other driving piston, toconnect the inside of cylinder 2 with an exhaust conduit il.

The compressor cylinders 5 are arranged respectively at the ends oicasing l5 and in each of said cylinders 4, the chamber adjacent to saidcasing I5 constitutes the compression chamber 6, the pre-compressionchambers 5 being then located each at one end of the machine.

Finally I provide a relatively important number (in order to facilitatethe circulation of air through the machine) of intake valves which arefitted at the free end of cylinder Il, discharge valves 9 which areprovided at the other end of said cylinder in such manner that they canconnect chamber 6 with the inside of casing i5, and finally valves Iadapted to cooperate with respective conduits 1 .which are provided inpiston 3 so as to permit air to ilow from chamber 5 to chamber with adirect flow.

Preferably piston i3 is made rigid with the end of cylinder i andcylinder IZ is provided in the corresponding reciprocating structure,these parts being so dimensioned that the pneumatic accumulators li canstore up a sufficient amount of energy, during the forward stroke of themov-- able elements, for ensuring, during the` return stroke of said.movable elements, the compression work in theengine Acylinder and mostof the air compression work -inthe compressor cylinders.

It should be notedl that, according to such an arrangement, valves ldhave a tendency to be applied on their seats by inertia before thestopping of the reciprocating eiementsfat Vthe' ends of their outwardstrokes. This drawback may be obviated either, as shown by liig.l 4, byplacing said valves under the dependence of springs su'ch as ld, whichtendv to keep them. open, or by providing at .the ends of cylinders dgrooves capable, when they are uncovered by pistons 3, to establish acommunication between chambers 5 and G close to the outerendsof thereciprocating d isplacements of said pistons.

An auto-generator made as above explainedy is such that air flows alwaysin the same direction which is substantially parallel to the axis of themachine, which improves the general efciency of said machine.Furthermore, the volumetric efliciency is high and varies but littlewhen the discharge pressure varies, which enables said generator to bealways 'capable of feeding receiving turbines, whatever be the load ofsaid turbines.

By way of example, for an auto-generator having the characteristicsabove mentioned for a.

` motor-compressor, the volumetric efficiency would vary only from 0.96toil for variations of the scavenging pressure from 5 kg. to 1 kg.,whereas the same pressure variations would produce variations of thevolumetric 'efliciency from A0.54 to l for a generator in which thecompression would be eiectedin the usual manner. l

Although it is possible to proceed as above explained, I have found thatit is preferable,

when -thecompressed air that is produced is capable of scavenging andfeeding the motor cylinder under variable pressures, which is the casewith auto-generators, to have further recourse to another arrangementaccording to the invention which consists in arranging in such mannerthat the power accumulators Il are of variable feed pressure. l

Furthermore, I provide means permitting to proportion, at leastapproximately, the amount of fuel injected into the motor cylinder tosaid feed pressure.

It is thus possible to proportion the energy produced in the motorcylinder to the energy that said pneumatic accumulators must be capableof giving up, at the time of the return stroke of the movable elements,this last mentioned amount of energy being essentially variable since itdepends not only upon the discharge stroke but also upon the dischargepressure, which varies when the machine is in operation.

For this purpose, it is advantageous to have recourse to one of theembodiments of the invention illustrated by Figs. 5 and 6.

I connect each pneumatic accumulator il with a chamber under pressure I8through a conduit I9 partly provided in piston i3 and said accumulatoris adapted to communicate with chainber l only during a portion (Welldetermined)` of the motion of the reciprocating elements, preferablyclose to the inner end of their reciprocating motion. For instance, Iprovide between said chamber I8 and said pneumatic accumulator li' avalve 2d the opening of which is controlled by a cam 2l rigid with oneof said reciprocating elements. Furthermore, advantageously, this camensures, by means of another inclined surface thereof, the operation ofthe feed pump 22.

' Chamber I8 is fed with compressed -air from a reservoir 23 containingairat a high pressure, for instance through an adjustable pressurerelief device including a valve 2d.

This device may be arranged, as shown by Fig. 5 in such manner that theworkman can adjust at lwill the expansion ratio by means of an operatinghandle 25. The handle is connected to a screw i2Y adapted to regulatethe `tension of a spring t3 tending to maintain the relief valve in aclosed position.

Alternately, in the embodiment illustrated by Fig. 6, the pressurerelief device is arranged in such manner that said expansion ratioisdeterminedv in an automatic manner, by a regulating device,advantageously as a function of the discharge pressure of the machine,said regulating device being then constituted, for instance, by a piston4S slidable in a cylinder 45 and acting on the spring 43- so as toregulate its tension. The cylinder 4l?4 communicates with the interiorof casing l5 by a conduit 4i. Preferably, the conduit I6 connecting therelief device with the valve 26 is provided with a constantly openleakage di.

The system is devised in such manner that the variationsy of the feedpressure of accumulators Il, resulting from the operation of thepressure relief valve 24, ensure variations in the same diact with thequantitative adjustment rod 2a or.

, pump 22.

This can be effected either in a direct manner, as shown by Fig. 5, saidrod being then controlled by a cam 3l, itself controlled by the rod 32of the piston.

Alternately, especially when rod 29 is already controlled by the rod 33of a regulating device. the same result can be ensured through a cor-/recting device which may consist. for instance as shown by Fig. 6, oftwo limiting inclined surfaces 3'1 and 35 pivoted about an axis 3G andcontrolled by said rod 32.

Advantageously, in vboth cases, a return spring 31 is provided forensuring the displacements of the piston in the direction correspondingto a reduction of the amount of fuel-that is injected.

In a general manner, while I have, in the above description, disclosedwhat I` deem to be practical and eillcient embodiments of the presentinvention, it should be well understood that I do not wish to be limitedthereto as there might be changes made in the arrangement, dispositionand form of the' parts without departing from the principle of thepresent invention as comprehended within-the scope of the appendedclaims.

What I claim is:

1. A free piston motor 'compressor unit comprising in combination, amotor cylinder, at least one motor piston adapted to slide freely insaid motor cylinder. compression means comprising at least onecompression cylinder and one compression piston operatively connected tosaid motor piston, said compression means forming a pre compressionchamber and a main compression chamber, the compression ratio in theprecompression chamber being a fraction of the compression ratio in themain compression chamber, valve means for admitting air in theprecompression chamber, one-way valve means for connecting theprecompression chamber with the main compression chamben'said last valvemeans being operable by pressure'in the precompression chamber to opencommunication with the main compression chamber and means fordischarging the compressed air out of said main compression chamber.

2. A free piston motor compressor unit according-to claim 1 wherein thecompressionA stroke in the precompression chamber coincides with theinlet stroke in the main compression chamber, and the compression strokein the main compression chamber coincides with the inlet stroke in theprecompression chamber.

3.4 A free piston motor compressor unit of the type describedcomprising, in combination, a motor cylinder at least one' compressioncylinder, a piston adapted to slide freely in said motor cylinder, asecond piston operatively connected to said first piston and adapted toslide in said cornpresslon cylinder, the piston sliding in saidcompression cylinder beingdouble-acting and dividing said, compressioncylinder in two compressor chambers, valvemeans for admitting a gaseousfluid in the ilrst of said chambers, one-way valve means for connectingsaid rst chamber with the second chamber, and means for discharging thecompressed fluid` out of said second chamber, the compression ratio insaid first chamber being lower than in said second chamber.

4. In a free piston motor compressor,`a mot-or cylinder, a motor pistonmovable therein, a compression cylinder, a compression piston movable insaid compression cylinder operatively connected to said inotor piston,said compression piston dividing said compression cylinder into aprecompression chamber and a compression chamber, one way valve means toadmit air to the precompression chamber, one-way valve means forconnecting said chambers to conduct air from the precompression chamberto the compression chamber, and means to discharge air from thecompression chamber.

5. A free piston compressor unit comprising in combination a motorcylinder, at least one motor piston adapted to slide freely in saidmotor cylinder, compression means comprising at least one compressioncylinder arid one compression piston operatively connected to said rstpiston and adapted to slide in said compression cylinder, saidcompression piston dividing said compression cylinder into a .maincompression chamber and a precompression chamber, valve means for admittlng a gaseous uid into the precompresslon chamber, pipe means forconnecting said precompression chamber and said compression chamber,said pipe means including a one-Way valve adjacent the entrance of saidcompression chamber, and means for discharging compressed fluid out ofthe compression chamber.

6. A free piston compressor unit comprising in combination a motorcylinder, at least one motor piston adapted to slide freely in saidmotor cylinder, compression means comprising at least one compressioncylinder and one compression piston operatively connected to said firstpiston and adapted to slide in said compression cylinder. saidcompression piston dividing said compression cylinder into a maincompression chamber and a precompression chamber, valve means foradmitting a gaseous fluid into the precompression chamber, means forconducting iiuid from said precompression chamber to said compressionchamber, means for discharging fluid out of said compression chamber,and means for conducting at least a partl of said uid to said motorcylinder, said cylinders and pistons being so arranged thatthe path ofthe gaseous fluid through said chambers and discharging means issubstantially in. a straight line.

7. A free piston compressor unit comprising in combination a motorcylinder, at least one motor piston adapted to slide freely in saidmotor cylinder, compression means comprising at least one compresmoncylinder aligned with said motor cylinder and one compression pistonoperatively connected to said rst piston and adapted to slide in saidcompression cylinder, said compression piston dividing said compressioncylinder into a main compression chamber on the same side of thecompression piston as the motor piston and a precompression chamber onthe opposite side thereof, valve means for admitting a gaseous fluid atthe outer end of the precompression chamber, valve means in saidcompression piston for conducting tluid from said precompression chamberto said compression chamber, means for discharging iluid out of saidcompression chamber, and means for conducting at least a part of suchfluid to said motor cylinder, said cylinders and pistons being soarranged that the path of the gaseous fluid through said chambers anddischarging means is substantially in a straight line.

8. A system as claimed in claim 6 wherein said path`is substantially'parallel to the axis of the machine. Y

9. A'free pistoncompressor unit comprising in combination a motorcylinder, at least one motor piston adapted to slide freely in saidmotor cylinder, means for returning saidpis'ton to its initial positionat the end of the expansion stroke. comaieasca pression `meanscomprising at leastone compression cylinder and one compression pistonoperatively connected to said rst piston and adapted to slide in saidcompression cylinder, said compression piston dividing'said compressioncylinder into a main compression chamber and a precompression chamber,valve means for admitting a gaseous fluid into the precompressionchamber, means for connecting said precompression chamber and saidcompression chamber, areceiver surrounding said motor cylinder", meansfor discharging compressed fluid out of the compression chamber intosaid receiver, and means for conducting at least a portion .cf the airfrom the 'receiver to the engine.

10. A free piston compressor unit comprising in combination a motorcylinder, at least one motor piston adapted to slide freely in saidmotor cylinder, compression means comprising at least one compressioncylinder and one compression piston operatively connected to said firstpiston and adapted to slide in said compression cylinder, saidcompression piston dividing said compression cylinder into a maincompression chamber and a precompression chamber, valve means foradmitting a gaseous fluid into the precompression chamber, valve meansin said compression piston fcrvconnecting said precompression chamberand said compression chamber, and means for discharging compressed fluidout of the compression chamber.

1i. In a device as claimed in claim l0 in which said compression pistoncauses compression in the precompression chamber on its outward stroke'and in the compression chamber on its inward stroke, said valve openinginto the compression chamber, and a spring acting on said valve in adirection to hold it open.

l2. A 'free piston motor compressor unit of the type describedcomprising, in combination, a motor cylinder, at least one compressioncylinder, a piston adapted to slide freely in said motor cylinder, asecond piston connected to said first piston and adapted to slide insaid compression cylinder, said second piston carrying out at leastl itsmain compression work on its inwardly ldirected stroke, means comprisingat least supplementary cylinder and piston elements one rigidlyconnected to the first mentioned cylinders and the other carried-by thefree piston, the supplementary piston fitting slidably in thesupplementary cylinder so as pensating pneumatic cushion that returnsthe free motor piston to its motor-compression position, and means forvarying the initial pressure in said compensating pneumatic cushion independence on the pressure of the'compressed fluid delivered by saidcompressor.

i3. A free piston motor compressor unit of the type describedcomprising, 'in combination, a motor cylinder, at least one compressioncylinder,

a piston adapted to slide freely in said motor cylinder, a second pistonconnected vto said rst piston and adapted to slide in said compressioncylinder, said second piston carrying ut at least its main compressionwork on its inwardly directed stroke, means comprising at leastsupplementary cylinder and piston elements one rigidly connected to thefirst mentioned cylinders and the other carried by the free piston, thesuppleslidably in the supplementary cylinder so as to form therewith acompensating pneumatic cushion that returns the free motor piston to itsmotor-compression position, a pump for delivering fuel to saidl motor toform therewith a com-I cylinder, a control element for said pump, meansfor varying the initial pressure in said compensating pneumatic cushion,and means responsive to variations in said initial pressure on saidpump-control element for increasing the quantity of delivered fuel withthe increase of said initial pressure.

14. A free piston motor compressor unit of the type describedcomprising, in combination, a motor cylinder, at least one compressioncylinder, a piston adapted to slide freely in said motor cylinder, asecond piston connected to said first piston and adapted to slide insaid compression cylinder, said second piston carrying out at least itsmain compression work on its inwardly directed stroke, means comprisingat least supplementary cylinder and piston elements one rigidlyconnected to the first mentioned cylinders and the other carried by thefree piston, the supplementary piston fitting slidably in thesupplementary cylinder so as to form therewith a compensating pneumaticcushion that returns the free motor piston to its motor-compressionposition, a pump for delivering fuel to said motor cylinder, a controlelement for said pump, means for actuating said control element,abutment means for limiting the displacements of said control element,means for varying the initial pressure in said compensating pneumaticcushion, and means responsive to variations in said initial pressure andacting on said abutment means to move the same.

l5. A free piston motor compressor unit of the type describedcomprising, in combination, a m0- tor cylinder, at least one compressioncylinder, a plston adapted to slide freely in said motor cylinder, asecond piston connected to said first piston and adapted to slide insaid compression cylinder, said second piston carrying out at least itsmain compression work on its inwardly directed stroke, means comprisingat least supplementary cylinder and piston elements one rigidlyconnected to the first-mentioned cylinders and the other carried by thefree piston, the supplementary piston tting slidably in saidsupplementary cylinder so as to form therewith a compensating pneumaticcushion that returns the free motor piston to its motor-compressionposition, a chamber containing a gas under pressure, means forconnecting said chamber with said supplementary cylinder at the end ofthe return-stroke of the supplementary piston sliding therein, and apressure reducing valve responsive to the pressure of. the compresseduid, for varying the pressure of gas admitted into said supplementarycylinder in accordance withv the effectivel value of said fluidpressure.

1`6. A free piston motor compressor unit of the type describedconiprising, in combination, a motor cylinder, at least one compressioncylinder, a piston adapted to inder, a second piston connected to saidfirst piston and adapted to slide in said compression cylinder, saidsecond piston carrying out at least its main compression work on itsinwardly directed stroke, means comprising at least supplementarycylinder and piston elements one rigidly connected to the firstmentioned cylinders and the other carried by the free piston, thesupplementary piston iitting slidably in the supplementary cylinder soas to form therewith a compensating pneumatic cushion that returns thefree motorl piston to its motor-compression position, a chambercontaining a gas under pressure, means for connecting said chamber 'withsaid supplementary cylinder at the end of the return-strokeof thesupplementary piston sliding therein, and a hand-operated pressurereducing ya1ve;for'varying the pressure of gas admitted in saidsupplementary cylinder.

17. A free piston motor compressor unit of the type describedcomprising, in combination. a. motor cylinder, at least one compressioncylinder, a piston adapted to slide freely in said motorcylinder, asecond piston connected to said first piston and adapted to slide insaid compression cylinder, said second piston carrying' outfat leasiritsmain compression Work on im inwardly directed stroke, means comprisingat least supplementary cylinder and piston elements one rigidlyconnected to the 'first mentioned cylinders and the other carried by thefree piston, the supplementary piston iltting sliciably in saidsupplementary cylinder 'so as to form therewith. a compensatingpneumatic cushion that returns the free motor piston toitsmotor-compression position, a chamber containing a gas under pressure,means for connecting said chamber with said supplementary cylinder atthe end of the return-stroke of the supplementary piston slidingtherein, a pressure reducing valve for varying the pressure o f gasadmitted into said supplementary cylinder, a pump for 'delivering fuelto said motor cylinder, a control element for said pump, and meansresponsive to variations of said pressure and acting on saidDump-control element to move the same.

RAUL PATERAS PESCARA.

